Eddy current probes for flaw detection in metals are well known. One type of prior art probe includes a single port, i.e., one set of terminals where both the excitation is applied and an impedance change or a resonant frequency change produced by the presence of a flaw is measured. Single port probes are shown, for example, in U.S. Pat. No. 4,286,216, issued Aug. 25, 1981, Auld et al., Great Britain Pat. No. 1,272,409, published Apr. 26, 1972, Shaternikov et el., and in articles by J. P. Watjen and A. J. Bahr, Evaluation of a Novel Eddy-Current Probe For Detecting Cracks Inside and at the Edges of Holes, "Review of Progress in Quantitative Nondestructive Evaluation, Vol. 2B," D. O. Thompson and D. E. Chimenti, eds., Plenum Press, New York (1983), pp. 1187-1202 and Evaluation of an Eddy-Current Tape-Head Probe, "Review of Progress in Quantitative Nondestructive Evaluation, Vol. 3A", D. O. Thompson and D. E. Chimenti, eds., Plenum Press, New York (1984), pp. 663-674.
Another type of probe, identified as a reflection probe, includes two sets of ports, one for excitation and one for detection. A prior art reflection type probe is illustrated in FIG. 1 of the drawings. Prior art reflection probes include differentially wound detection coils such that in the absence of a flaw, equal voltages of opposite phase are induced therein. The presence of a flaw perturbs the voltage in one pickup, or detection, coil to a greater extent than the voltage change in the other detection coil for a change in the output from the detection port.